1. Field of the Invention
The present invention relates to a detection device, an exposure apparatus, and a device manufacturing method using the same.
2. Description of the Related Art
There has been used a projection exposure apparatus (exposure apparatus) that projects a pattern written on a reticle onto a wafer or the like and transfers the pattern thereto via a projection optical system as an apparatus for manufacturing a device (e.g., a semiconductor element, a liquid crystal display element, or a thin-film magnetic head) using a lithography technique. At this time, the projection exposure apparatus performs exposure after a pattern transfer region present on a wafer is aligned in a position at which a reticle pattern image is formed using an alignment detection system (detection device) installed in the interior of the projection exposure apparatus.
In recent years, special elements such as stacked devices using a through-VIA such as MEMSs, CMOS image sensors (CIS), and the like in addition to IC chips such as conventional memories and logics have come to be used and manufactured by using an exposure apparatus. Elements such as MEMSs, CISs, and the like differ at some point from IC chips. In the elements such as MEMSs, CISs, and the like, line width resolution and superposition accuracy required for IC chips are not demanding but a deep focal depth is required. Also, as a special step of manufacturing elements such as MEMSs, CISs, and the like, there is a step of forming an alignment mark on the back side of a Si wafer and then exposing the front side of the Si wafer while being aligned with the mark provided on the back side of the Si wafer. An exemplary representative application is that, after thinning the Si wafer, a through-VIA is formed from the front side of the Si wafer to thereby be conductively communicatable with the circuit provided on the back side of the Si wafer. Japanese Patent Laid-Open No. 2002-280299 discloses a lithography apparatus that forms an image of the alignment mark formed on the back side of a Si wafer on the front side thereof and then detects the position of the alignment mark on the front side of the Si wafer using an alignment detection system configured on the back side (wafer chuck side). However, in the lithography apparatus including an alignment detection system configured on the back side of a Si wafer, a hole is provided at a specified position of a wafer chuck, and thus, only the alignment mark at the specified position can be detected. Thus, in the method disclosed in Japanese Patent Laid-Open No. 2002-280299, an alignment mark which is arranged at any position over the back side of a wafer cannot be observed.
Here, the Si wafer is transmissive in respect of infrared light (wavelength of 1,000 nm or greater). In recent years, in contrast to the configuration disclosed in Japanese Patent Laid-Open No. 2002-280299, there has been proposed a method for observing the alignment mark formed on the back side of the Si wafer from the front side thereof using a position detection system which uses infrared light as a light source. In this case, in the normal alignment sequence, in order to measure the best focus position of the alignment mark, an image of the alignment mark is acquired while the wafer stage is being driven in the optical axis direction of the alignment detection system to thereby calculate the position with highest contrast. Hereinafter, such a measuring method is referred to as “image autofocus measurement”. In the image autofocus measurement, measurement is started from the height of the reference plate, so that the alignment mark formed on the front side of the Si wafer can be quickly and readily detected. However, when there is an alignment mark formed on the back side of the Si wafer, the alignment detection system is in-focus at the reference plate during a normal operation, resulting in the failures below. For example, if the wafer stage is driven from the reference plate which is the default focus position of the alignment detection system, a large search area needs to be taken for detecting the alignment mark provided on the back side of the Si wafer. At this time, increasing the search area means that more time is required for measurement, resulting in a reduction in throughput. Furthermore, when the measurement pitch of the image autofocus measurement increases, the computed errors of the best focus position of the alignment mark increase, so that highly-accurate alignment cannot be achieved.
On the other hand, in order to form the through-VIA from the front side of the Si wafer to thereby be conductively communicatable with the circuit provided on the back side of the Si wafer, the Si wafer needs to be thinned in advance by a wafer thinning device. At this time, the thinning device firstly determines processing conditions for achieving a desired wafer thickness and then performs thinning of the Si wafer. However, the thickness of the thinned Si wafer is readily varied due to the change over time or the like of the thinning device. For example, when a photo sensor is produced via a back side illumination (BSI) process, the variation in thickness of the wafer affects the characteristics of the photo sensor, the wafer thickness management is particularly important. The variation in thickness of the wafer may affect not only the detection of the alignment mark formed on the back side of the Si wafer but also wafer handling. For example, the thinned Si wafer exhibits a weak mechanical strength, resulting in a high cracking tendency during handling. Furthermore, in the through-VIA step, the through-VIA is formed by etching the Si wafer subjected to thinning. However, if there is variation in thickness of the Si wafer, the VIA formed by etching may not properly penetrate the Si wafer. In contrast, if the etching time by the etching device is too long in order to surely penetrate the VIA, the VIA may also penetrate an etching stop layer, resulting in a destruction of the device itself underling the etching stop layer. Consequently, the variation in thickness of the Si wafer may affect the yield of devices to be manufactured. Accordingly, if the thickness of the Si wafer can be detected by the alignment detection system in the exposure apparatus, the variation in thickness of the Si wafer can be feedback to the thinning device by automatically and periodically monitoring the thickness of the Si wafer. Furthermore, if the detected wafer thickness is feedback to the etching device, the through-VIA can be formed at the optimum etching time, resulting in an improvement in yield of devices.